1. Field of the Invention
The present invention relates, in general, to switching mode power supplies and power conversion system, and in particular to methods and apparatuses for power conversion with wide input voltage range. The present invention can be used in a standalone converter or as part of a larger system. For example, one application of the present invention is to provide an apparatus that can increase the output voltage holdup time in off-line power supplies during a loss of input power.
2. Description of the Related Art
Power devices such as MOSFETs, for example, used in known switching mode power supplies (SMPS) and converters work as high frequency (typically hundreds of kilohertz) ON/OFF switches. In the ON state, the switch can conduct a relatively large current at near zero voltage across its terminals, resulting in a low power loss. In the OFF state, a relatively large voltage can be applied to the switch, but the current is near zero, resulting again in a very small power loss. Power losses during the ON/OFF transitions, also known as switching losses, can be reduced using conventional techniques. As a result, SMPS and power converters are widely used in telecommunications, industrial, and other applications because of their high efficiency and excellent thermal performance.
In order to have the output voltage regulated, the ratio of ON time of the power switches to the switching period, called duty ratio or duty cycle, is typically controlled by feedback circuitry. The duty cycle range required to keep output voltage constant is typically on the same order as the input voltage range. For example, the output voltage, Vo, of an ideal forward converter is proportional to the input voltage Vin and to the duty cycle D and is inversely proportional to a turn ratio TR of a power transformer (the number of primary turns divided by the number of secondary turns):
      V    0    =            (                        V          in                TR            )        ×          D      .      
To ensure that the output voltage is regulated in the full input voltage range Vin max/Vin min, the duty cycle range Dmax/Dmin based on the equation above must be equal to the voltage range:
            D      max              D      min        =                    V                  in          ⁢                                          ⁢          max                            V                  in          ⁢                                          ⁢          min                      .  
Because the minimum pulse width is limited in practice, the input voltage range is limited. For example, if the switching frequency is 250 kHz and Dmax=50%, then the maximum ON time is 2 μs. If the minimum ON time is 0.5 μs, then Dmin=(minimum ON time)/(switching period)=0.5/4 or 12.5%. Thus, the voltage input range is limited to Vin max/Vin min=Dmax/Dmin≦(50/12.5)=4.
When a wider input voltage range is required, a different technique must be used. One conventional technique is to reduce the switching frequency that is associated with the larger magnetic components. This results in larger filters and consequently larger converter sizes. Another known technique is shown in FIG. 1. The block diagram shown in FIG. 1 includes N power converters connected in parallel on input and output sides. Each of the power converters covers a relatively narrow input voltage range, and the rest of power converters are off in that particular range. Consider for example a parallel structure of three power converters connected according to FIG. 1, with Vin min=9 V, Vin max=72 V. Let the first power converter be active in the 9 V to 18 V input voltage range, the second power converter be active in the 18 V to 36 V input voltage range, and the third power converter be active the in 36 V to 72 V input voltage range. In this particular example, the parallel combination of the three power converters covers the input voltage range Vin max/Vin min equal to 72 V/9 V=8, while each power converter only works in the range equal to 18 V/9 V=36 V/18 V=72 V/36 V=2.
An advantage of this known parallel structure is high efficiency because each power converter can be optimized in a relatively narrow input range. A disadvantage of this known parallel structure is an increased number of power converters, where each power converter is designed for full power, which leads to increased size and cost of the power conversion system.
The above demonstrate that there is an unmet need in the field for a more effective method and apparatus suitable for power conversion with wide input voltage range.